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air fuel ratio can change by changing the fuel pressure. higher pressure will make the engine run richer and lo pressure will make it lean. upgrading the fuel pump to a higher flow should not make any changes to the afr, but if the new pump produces more pressure than the old pump, the final pressure to the fuel rail will be higher than before even if you did not adjust the fuel pressure regulator, which in turn will make the engine run richer. the only reason i use adjustable fuel pressure regulator on turbo engines is because i like to run 75 psi of fuel pressure.
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